| Lithium-ion cells are considered presently the best choice for rechargeable batteries. Lithium-ion cells have many advantageous compared with other secondary batteries. Lithium-ion battery has high voltage, high energy density, long cycle life, self-discharge rate is small, no memory effect and also the electrode material does not contain toxic substances, is the modern "green battery." It is widely used in mobile phones, notebook computers, electric vehicles and hybrid electric vehicles. Since the first commercialization of Li-ion batteries by Sony in 1991, graphite carbon has been the favorable anode material for its good reversibility and stability with thousands of cycles. However since the theoretical capacity (372 mAh g-1) of graphite is limited, new anode materials with high specific capacity are searched to satisfy the requirement of advanced power sources in such applications as electric vehicles with extended range.The search for next-generation anode materials of Li-ion batteries has focused on Si- and Sn-based oxide materials that offer a considerably larger specific (4200 mAh g-1 and 994 mAh g-1) and volumetric capacity than conventional carbonaceous materials. Such studies indicate that silicone monoxide, SiO, has a large discharge specific capacity. However, due to their serious volume change when charging and discharging, leading to fast capacity fading and poor cycle performance, it is a great impact to the material of its practical value. The composite of Si- and Sn-based compounds or alloys can effectively improve their cycling performance. In this work, we studied Si-SiOx-Sn/C composite which was prepared by thermal reduction method, and tested it as anode material for Li-ion battery. The composites were synthesized by heating the milled mixture of SiO, SnO2 and carbon in an inert gas filled furnace. The effects of temperature, electrochemical properties and impedance were researched, and the results were as follows:1. Si-SiOx-Sn/C composites were prepared in following steps:the mixture of SiO powder, carbon powder and Nano-SnO2 powder was milled in a planetary ball mill for 5 h. The milled composites were dispersed into the acetone solvent. The dried solid mixtures were heated to 750℃,800℃,850℃,900℃,950℃and 1000℃for 2 h respectively in a furnace tube under an inert atmosphere. The XRD results show that SnO2 can be reduced into Sn metal above 800℃, and silicon-mono-oxide is transformed into silicon and silicon dioxide (2SiO→Si+SiO2). Further results indicated that the composite heated at 900℃for 2 h is better in performance.2. Charge and discharge measurements were carried out employing a two-electrode 2025 coin cell. Lithium metal was used as the counter electrode. When the composite was heated at 900℃for 2 h, the first discharge capacity of the composite anode can reach as high as 1144.2 mAh g-1 under a constant current density of 225 mA g-1.3. Further results show that the first discharge capacity of the composite anode heated at 900℃for 2 h can reach 1144.2 mAh g-1,715.8 mAh g-1,359.0 mAh g-1 and 282.9 mAh g-1 under 0.5 C,1 C,5 C,10 C.4. We researched the typical EIS Nyquist plots of composite anode heated at 900℃, after the first, third and 30 th cycles. The diameter of the semicircle becomes larger with the increase of cycle number, which indicates that the charge transfer reaction process becomes more difficult as charge and discharge cycles proceed. Increase of interface and particle contact resistance could be a cause of the capacity decay. |
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